But perhaps the best is this offering from the Chinese Women’s Association of San Diego, via Stuff Asian People Like:

Aiya is an all-purpose phrase that comes from deep in the soul. Aiya is both simple and complex: on one hand it is a couple of Chinese characters, on the other hand it can be a whole speech describing the state you are in. Aiya says, “I’m afraid”, “I’m in pain”, “I don’t believe it.” It is an exclamation of exuberance, a shout of hurt, a cry of fear, and the reflex of being startled, and the embrace of joy.